(a) Field of Invention
The invention relates to a method for crushing material of different grain size, in which the material is supplied to a rotating, horizontally positioned grinding surface of a grinding-classifying chamber having a casing wall and is crushed to grinding material particles, in which the grinding material particles are supplied with the aid of a delivery flow introduced on the circumference of the grinding surface to a classifying process and fine material particles are discharged and in which a part of the oversize occurring as coarse material particles is removed, as well as to an apparatus for performing the method especially an air-swept mill having between a rotating grinding pan and a casing wall an annular space with a blade ring for a fluid delivery flow.
(b) State of Prior Art
It is known to relieve a grinding-classifying chamber, e.g. an air-swept roller mill, of the non-crushed or not adequately crushed grinding material, the so-called oversize, in that said oversize is drawn off to the outside during the grinding process and is then generally returned to said process.
Through the removal of a specific percentage of oversize material, the flow resistance in the grinding-classifying chamber drops, so that it is possible to reduce the delivery energy for a fluid flow to be expended on a unit. The necessary mechanical energy for a mechanical oversize return located outside the grinding-classifying chamber is much smaller.
DE 41 24 416 A1 discloses such a method. Oversize material, which has been rejected by the classifier as tailings, on a dropping path to a grinding surface are at least partly removed to the outside via a screw conveyor.
According to a method known from DE-AS 1 152 297 in an air-swept roller mill with integrated classifier a fluid, e.g. air or a gas is introduced into a grinding-classifying chamber. The fluid is brought in an annular space between the grinding pan and the casing wall to such a high speed that substantially all the grinding material particles spun off from the grinding pan under the action of centrifugal force, i.e. from the charge particle size to the finished particle size, are taken up by the fluid flow and conveyed to the classifier as a two-phase mixture.
By means of a fluid flow at a relatively low speed it is ensured that a high percentage of largely non-crushed material drops downwards out of the mill. In this way the pneumatic conveying energy of the fluid flow unit can be lowered as a result of a reduced flow resistance due to a lower grinding material load.
In the known methods grinding material particles are drawn from the grinding, classifying, drying and pneumatic conveying process occurring the mill and the classifier, so that there is a change to one of the numerous factors influencing a complex dynamic grinding-classifying system.
It is known that the modification of one factor, e.g. the modification to the speed of the delivery flow in the annular clearance of the removal of grinding material particles from the grinding-classifying process, influences further parameters, such as e.g. the wall friction, gas friction, friction between the gas and grinding material particles, flow formation in the mill and classifier, particle distribution and particle sizes, so that there must be a new state of equilibrium.
Even a partial removal of uncrushed or inadequately crushed grinding material particles has a negative influence on the grinding bed formation on the grinding pan, because coarse particles in conjunction with fine particles lead to a maximum packing density and to an almost ideally compacted and optimum crushable grinding bed. However, if coarse particles, e.g. up to 250%, based on the finished material flow, is removed from the grinding material flow enriched with particles and circulating in the grinding-classifying chamber, the coarse particles are more particularly missing from the grinding bed formation. This leads to the disadvantage of there being no autogenous grinding aid on the part of the coarse particles. Since during the external delivery process no crushing occurs, neither the coarse nor the particles undergo further crushing.
Although a reduction of the flow energy leads to a saving of pneumatic delivery energy at the fan, this procedure leads to a reduced crushing capacity, so that all things considered there is no saving with respect to the overall, specific power requirement.